Don't bother with a big valve head...it's plenty big enough for the 2.0. If you want a quick and easy upgrade, drop in the cam from a 740 injection (B200E or B230E) -stamped with a V. It'll give you a few extra horses (122?), and they're everywhere.
If you want a genuine upgrade, by far the easiest thing is to drop in a 2.3 lump. The B230F from a late 740 or early 940 looks identical to a 360 engine (insurance

), plus has lambda fuel control for better emissions and economy. Rip the cat out, put a hotter cam back in (the 'A' from the GLT, or the 'V' from the 740 injection), and you've got the power back without losing efficiency.
An added bonus would be a B230FB (early 940 again), which is a higher performance version with a different cam and a better flowing head as standard - and of course the ECU is mapped exactly right for that configuration.
cheers
James